When a landscape becomes fragmented, the patches that remain vary in how far apart they are and whether they’re good or bad quality. When individuals try to disperse among these patches, they have to decide both whether to stay or leave their current patch (emigration), as well as where to go if they decide to leave (immigration).
If we understand what affects these dispersal choices, we can better predict how individuals will move in fragmented habitats and what the ultimate ecological and evolutionary consequences might be. Many dispersal studies focus on large, mobile species in natural landscapes or seascapes, but there are other ways to answer questions about how and why individuals move. A recent study takes a novel approach with a small-scale, experimental corridor set-up that uses a microbe to test how habitat fragmentation affects dispersal.
Tetrahymena thermophila is a microbe that lives in North American freshwater ponds and streams, and is an important consumer of bacteria. It adjusts its dispersal decisions based on temperature and, like many species, on the presence of food resources. It can be distinguished as “individuals” based on genotype, and movement can be tracked based on concentration in an aqueous solution.
Experimental corridors can be highly useful to test for meaningful effects of fragmentation. This study used an experimental set-up of one central, occupied tube connected to an unoccupied tube on either side via a corridor of plain water to represent habitat patches connected by a harsh matrix. The two side patches varied in temperature extremes (27°C vs. 35°C) to represent the difference between optimal and suboptimal habitat. This set-up was constructed with both “standard” length corridors (5 cm) and with corridors twice as long (10 cm), and replicated multiple times with six different genotypes of T. thermophila. After enough time had elapsed to allow for movement, the number of cells were counted in both the central patch and neighboring patches.
Some results were as expected, but others were surprising. As expected, two genotypes significantly reduced their dispersal rate when facing longer corridors. Increasing the distance between patches limited habitat choice at immigration (i.e. choosing where to settle): the genotypes that performed habitat choice at immigration with standard length corridors did not significantly differ from random dispersal when there were longer corridors.
Increased distance between patches led to an overall increase of habitat choice at emigration, suggesting that genotypes become choosier in their decision to either stay or leave their patch when obtaining information about neighboring patches gets harder. However, 3 of the 4 genotypes that were choosier ended up preferring less optimal habitats, a strategy that could potentially become maladaptive.
While it seems straight-forward that habitat fragmentation would affect dispersal rate, this experiment shows that it can also impact whether individuals choose to disperse and how they choose where to go. This can have long-term, large-scale ecological and evolutionary impacts. The movement of species between habitat patches is particularly important to understand given the predicted large-scale land use change and climate change of the future.